Semiconductor material and method for forming the same and thin film transistor

ABSTRACT

A semiconductor material and a method for forming the same, said semiconductor material having fabricated by a process comprising irradiating a laser beam or a high intensity light equivalent to a laser beam to an amorphous silicon film containing therein carbon, nitrogen, and oxygen each at a concentration of 5×10 19  atoms·cm −3  or lower, preferably 1×10 19  atoms·cm −3  or lower, without melting the amorphous silicon film. The present invention provides thin film semiconductors having high mobility at an excellent reproducibility, said semiconductor materials being useful for fabricating compact thin film semiconductor devices such as thin film transistors improved in device characteristics.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor material, e.g.,containing silicon as the major component. More particularly, thepresent invention relates to a thin film silicon semiconductor materialimproved in properties and a process for fabricating the same. Thesemiconductor material according to the present invention enablesfabrication of thin film semiconductor devices such as thin filmtransistors having excellent device characteristics.

[0003] 2. Description of the Prior Art

[0004] Non-crystalline semiconductor materials (the so-called amorphoussemiconductors) and polycrystalline semiconductor materials have beenheretofore used for the fabrication of thin film semiconductor devicessuch as thin film field effect transistors and the like. The term“amorphous materials” as referred herein signifies not only thematerials having a structural disordering in the strict sense of atomiclevel, but also includes those having a short range ordering for adistance of about several nanometers. More concretely, “amorphousmaterials” include silicon materials having an electron mobility of 10cm²/V·s or lower and materials having a carrier mobility lowered to 1%or less of the intrinsic carrier mobility of the correspondingsemiconductor material.

[0005] The use of an amorphous semiconductor such as amorphous silicon(a-Si) and amorphous germanium (a-Ge) in the fabrication ofa-semiconductor device is advantageous in that the process can beconducted at a relatively low temperature of 400° C. or even lower.Thus, much attention is paid now to a process using an amorphousmaterial, because such a process is regarded as promising for thefabrication of liquid crystal displays and the like, to which a hightemperature process cannot be applied.

[0006] However, a pure amorphous semiconductor has an extremely lowcarrier mobility (electron mobility and hole mobility). Thus, pureamorphous semiconductors are seldom applied as they are, for example, tochannel-forming areas of thin film transistors (TFTs). In general, thepure amorphous semiconductor materials are subjected to the irradiationof a high energy beam such as a laser beam or a light emitted from axenon lamp, so that they may be once molten to recrystallize and therebybe modified into a crystalline semiconductor material having an improvedcarrier mobility. Such a treatment of high energy light beam irradiationis referred hereinafter collectively to as “laser annealing”. It shouldbe noted, however, that the high energy beam not necessary be a laserbeam, and included in the high intensity beam is, for example, apowerful light emitted from a flash lamp which has a similar effect onthe semiconductor material as the laser beam irradiation.

[0007] Generally, however, the semiconductor materials heretoforeobtained by laser annealing are still low in the carrier mobility ascompared with those of single crystal semiconductor materials. In thecase of a silicon film, for example, the highest reported electronmobility is 200 cm²/V·s at best, which is a mere one seventh of theelectron mobility of a single silicon, 1350 cm²/V·s. Moreover, thesemiconductor characteristics (mainly mobility) of the semiconductormaterial thus obtained by the laser annealing process suffers poorreproducibility and also scattering of the mobility values over thesingle film. Those disadvantages lead to a low product yield ofsemiconductor devices having a plurality of elements fabricated on asingle plane.

SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide a semiconductormaterial having a high mobility and a method for forming the same withexcellent reproducibility. More specifically, an object of the presentinvention is to provide a process in which the problems of theconventional laser annealing process are overcome, and to provide,accordingly, a practically feasible thin film semiconductor materialimproved in characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a graph showing the relation between the centerwavenumber of the Raman peak (Raman shift; taken in the abscissa) andthe electron mobility (the ordinate) of a laser-annealed silicon film.The oxygen concentration of the film is found to be 2×10²¹ atoms·cm⁻³;

[0010]FIG. 2 is a graph showing the relation between the centerwavenumber of the Raman peak (Raman shift; taken in the abscissa) andthe electron mobility (the ordinate) of laser-annealed silicon filmswith varying oxygen concentration;

[0011]FIG. 3 is a graph showing the relation between the ratio of thefull band width at half maximum (FWHM) of the Raman peak for alaser-annealed silicon film to the FWHM of the Raman peak for a singlecrystal silicon (FWHM ratio; taken in the abscissa) and the electronmobility (the ordinate), for laser-annealed silicon films with varyingoxygen concentration;

[0012]FIG. 4 is a graph showing the relation between the peak intensityratio (Ia/Ic; taken in the abscissa) and the electron mobility (theordinate) for laser-annealed silicon films with varying oxygenconcentration, where Ia represents the Raman peak intensity (of the peakat a wavenumber of about 480 cm⁻¹) for the amorphous component of thelaser-annealed silicon film, and Ic represents the Raman peak intensity(at about 521 cm⁻¹) for the single crystal silicon;

[0013]FIG. 5 is a graph showing a position-dependent change of FWHM ofthe Raman peak for channel-forming areas of field effect transistors,where, the abscissa is X/L with L representing the channel length, andthe ordinate is FWHM; and

[0014]FIG. 6 shows a process for fabricating a field effect transistor.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Raman spectroscopy is an effective method for evaluating thecrystallinity of a material, and it has been used with the purpose ofquantitatively evaluating the crystallinity of a semiconductor filmfabricated by a laser annealing process. During an extensive study onthe laser annealing process conducted by the present inventors, it hasbeen found that the center wavenumber, as well as the width, the height,etc., of the Raman peak of a laser-annealed semiconductor film isclosely related with the properties of the semiconductor film.

[0016] For instance, the Raman peak at 521 cm⁻¹ for a single crystalsilicon was observed on a laser-annealed silicon film to be shifted to ashorter wavenumber (longer wavelength). It has been also found that thecenter wavenumber of this Raman peak is strongly correlated with thecarrier mobility of the silicon film obtained by laser annealing.

[0017] Referring to FIG. 1, an example which illustrates the relationabove is explained. FIG. 1 relates the center wavenumber of the Ramanpeak (abscissa) to the electron mobility (ordinate) of a film obtainedby laser-annealing an amorphous silicon film. The electron mobility wasobtained by measuring the capacitance-voltage (C-V) characteristics on aTFT having fabricated from the silicon film. From FIG. 1, it can be readthat the electron mobilities for those having a Raman peak center of 515cm⁻¹ or higher behave quite differently from the mobilities of thosehaving a Raman peak center below 515 cm⁻¹. More specifically, it can beseen that the Raman peak center is more sensitive to the change inelectron mobility in the peak center wavenumber region of 515 cm⁻¹ orshorter; beyond this wavenumber, in contrast, a little shift of Ramanpeak to higher wavenumber side signifies a large increase in electronmobility.

[0018] This phenomena is clearly an evidence of the presence of twophases. According to the study of the present inventors, the phaseobserved with a Raman peak at 515 cm⁻¹ or lower is assumed as a phasewhich has achieved atomic ordering in the solid phase without undergoingmelting, whereas the phase having a Raman peak of 515 cm ¹ or higher isassumably a phase having solidified from a liquid phase which has onceexperienced melting by laser annealing.

[0019] The center wavenumber of the Raman peak was 521 cm⁻¹ at maximum,and the highest observed electron mobility was about 200 cm²/V·s.However, it was difficult to obtain such silicon films of high electronmobility with favorable reproducibility. Even though the laser annealingwere to be conducted under apparently the same conditions, most of theresulting silicon films suffered a low electron mobility of less than100 cm²/V·s, probably due to a slight difference in the crystallinestate. Those films with low electron mobilities yielded Raman peaks withtheir center at significantly lower wavenumber than 521 cm⁻¹, mostly at515 cm⁻¹ or lower.

[0020] The fact that 200 pieces of amorphous silicon film havinglaser-annealed under the same conditions fail to yield the same mobilityis another evidence which illustrates the problem of low reproducibilityof high electron mobility. Among the 200 pieces subjected to laserannealing, 3 pieces yielded a mobility of 200 cm²/V·s or higher, 11pieces yielded a mobility between 100 cm²/V·s and 200 cm²/V·s (200cm²/V·s excluded), 61 pieces yielded a mobility between 10 cm²/V·s and100 cm²/V·s (100 cm²/V·s excluded), and 125 pieces yielded a mobility ofless than 10 cm²/V·s.

[0021] The reason for such a scattering in the values of electronmobility is believed to be due to the fluctuation in laser output perpulse, and to the extremely narrow range of the optimum condition forthe laser annealing. According to an observation, for example, a toohigh laser output produces amorphous materials instead of effectingfavorable recrystallization.

[0022] In addition to the reasons above, the present inventors tooknotice of foreign atoms such as oxygen, nitrogen, and carbon atomshaving incorporated in the film. In the laser-annealed silicon filmsinvestigated and yielding the results shown in FIG. 1, oxygen atoms werefound to be present in the film at a concentration of 2×10²¹ atoms·cm⁻³.These oxygen atoms are believed to have been incorporated in some wayduring the deposition of the amorphous silicon film. Nitrogen and carbonatoms were present in trace amounts. Thus, an amorphous silicon film wasdeposited using gas materials of high purity while maintaining thedeposition chamber, the evacuation systems, and the like at asufficiently clean state. Under such a highly clean condition, littleamount of oxygen was added intentionally to the atmosphere to controlthe amount of the oxygen incorporated into the film, and the resultingfilm was laser-annealed to see the relationship between the center valueof the Raman peak and the electron mobility.

[0023] The concentration of elements other than silicon, such as oxygen,nitrogen, and carbon, is referred hereinafter to the concentration ofthose elements at the central portion of the film. The concentration ofthe elements in the central region is taken into account because despitethe extremely high concentration of those elements in the portion nearthe substrate or at the vicinity of the surface, the impurity elementsin those portions are believed to have little influence in the carriermobility which is to be considered in the present invention. In general,within a coating, the portion low in concentration of those foreignelements exists at the central portion of the film, and it is believedthat the central portion of the film plays an important role in asemiconductor device such as a field effect transistor. Accordingly, “aconcentration of a foreign element” referred simply herein is defined asthe concentration at the central portion of the film.

[0024] The influence of oxygen concentration on the relation between theelectron mobility and the Raman peak center is illustrated in FIG. 2.Obviously, the electron mobility is significantly improved by reducingthe oxygen concentration of the film. The same tendency was observed forboth carbon and nitrogen. As a possible explanation for the resultsobtained above, the present inventors propose two mechanisms as follows.In the case of laser annealing a film containing oxygen at a highconcentration, the portions low in oxygen atoms serve as crystal nucleito effect crystal growth during the melting and recrystallization of thefilm; the oxygen atoms incorporated into the film are driven to theperiphery with the growth of the crystal, and are precipitated at thegrain boundaries. Thus, the film as a whole which results from such aprocess is lowered in mobility due to the barriers created at the grainboundaries. Another possible explanation the present inventors proposeis that the oxygen atoms or the areas rich in oxygen (in general, thoseareas are believed to have a higher melting temperature) function ascrystal nuclei to effect crystal growth. Accordingly, the size of theindividual crystals becomes smaller with increasing population of theoxygen atoms. This leads to a lower crystallinity because of thecrystals having grown into smaller size, which in consequence give alower mobility.

[0025] At any rate, decreasing the oxygen concentration in the film iseffective for significantly increasing the electron mobility thereof bylaser annealing. For example, an electron mobility as high as 1000cm²/V·s was obtained by controlling the oxygen concentration to 1×10¹⁹atoms·cm⁻³. Similar results were obtained by lowering the concentrationsof nitrogen and carbon atoms as well. Also, a similar tendency wasobserved on hole mobilities.

[0026] Furthermore, as in the case of FIG. 1, the curve relating theelectron mobility to the Raman peak position shows typically aknickpoint (i.e., a bent) irrespective of the oxygen concentration.Referring to FIG. 2, the present inventors denoted the region at theright hand side (the side higher in wavenumber) of the broken lines as amelting and crystallizing region. This is because a film falling in thisregion is surmised that it has undergone melting and recrystallizationupon laser annealing. A high mobility was obtained on films falling inthis region.

[0027] However, there was no particular improvement with respect toreproducibility. For example, only 9 pieces out of 100 amorphous siliconfilms having fabricated with the oxygen concentration controlled to1×10¹⁹ cm⁻³ or lower yielded an electron mobility higher than 1000cm²/V·s, despite the laser annealing was conducted on each film undernominally the same conditions. Upon observation of the laser-annealedfilms, many were found to recover the amorphous state due to the failureof crystallization with too high laser output.

[0028] As described above, it is quite difficult to obtain practicallyfilms falling in the melting and crystallizing region, i.e., the righthand side of the broken line shown in FIG. 2. Moreover, it is notfavorable to try to obtain films in this region, because failure morelikely results from such an attempt.

[0029] A semiconductor material in accordance with the present inventionpreferably contains oxygen, nitrogen and carbon at a concentration of5×10¹⁹ atoms·cm⁻³ or less respectively therein.

[0030] A semiconductor material in accordance with the present inventionhas a Raman peak center preferably at a Raman shift of 517 cm⁻¹ or less,more preferably 512 cm⁻¹ or less.

[0031] The present inventors found that a high mobility can be obtainedby reducing the concentrations of oxygen, nitrogen, and carbon. Forexample, as shown in FIG. 2, a high electron mobility of 100 cm²/V·s wasobtained by simply reducing the oxygen concentration to 1×10¹⁹ cm⁻³ orlower. In addition, a mobility in this level can be readily obtained;for example, 72 films out of 100 amorphous silicon films havinglaser-annealed under the same condition yielded a mobility not lowerthan 80 cm²/V·s.

[0032] The present inventors assume that the amorphous silicon filmattains, without undergoing melting, some long-range periodicity in thisregion ascribed to a progressive lattice ordering in a solid phase orduring the intermediate state between solid and liquid phases.Accordingly, the present inventors denoted this region as a “solid phaseordering region”. Despite it is yet to give a definite reason for thehigh mobility of the films falling in this solid phase ordering regionwith low concentrations of oxygen, nitrogen, and carbon, a possibleexplanation the present inventors provide is as follows.

[0033] While it is true that the films falling in this solid phaseordering region do not experience a melting process, the atoms in thefilm move upon absorption of the light energy or the thermal energysupplied by the laser beam to attain a state of lowest energy, i.e., acrystalline state. However, since the film does not undergo a moltenstate, a completely crystalline state can not be achieved. Thus, someregions having an ordering at a range of from several to several tens ofnanometer develop within an amorphous matrix. Such a state is clearlydistinguished from a polycrystalline state which is achieved through anordinary molten state. That is, in the ordinary process of melting andcrystallization, crystal nuclei which form in the liquid phase collidewith each other during their growth into larger crystals. Such collisioninterfaces become the grain boundaries and consequently barriers for thecarriers, because lattice defects and impurities concentrate, ionizationoccurs to polarize, etc.

[0034] In the case of solid phase ordering, on the other hand, noprecipitation of impurities occur in the grain boundary since nocollision occurs between the crystals. Thus, the barriers between theordered regions such as the crystals are assumably very low. Thepresence of a foreign element which impairs the semiconductorcharacteristics is, then, the determining factor of the electricproperties of a semiconductor having obtained by a solid phase ordering.

[0035] The fact above can be clearly read from FIG. 2. For example, afilm having an oxygen concentration of 1×10¹⁹ cm⁻³ or lower gives anelectron mobility of about 100 cm²/V·s. Referring to the centerwavelength of the Raman peak, however, it can be seen that the peak islocated far from that of the single crystal silicon, 521 cm⁻¹. This factshows that the crystallinity of this film is not necessarily approachingthat of a single crystal silicon. This point will be further clarifiedby other data which will be given later.

[0036] A similar tendency was observed by the present inventors on therelation between the full band width at half maximum (FWHM) of the Ramanpeak and the electron mobility. The relation is shown in FIG. 3. Theabscissa is the full band width at half maximum ratio (FWHM ratio),which is defined herein as a ratio of the FWHM of a Raman peak obtainedon a laser-annealed film to that obtained on a single crystal silicon. Asmaller FWHM ratio closer to unity represents a film having a structuresimilar to that of a single crystal silicon. Referring to FIG. 3, it canbe seen that the FWHM ratio becomes closer to 1 with increasing electronmobility if films of equal oxygen concentration are compared. Also, aswas the case above in the relation between the mobility and the Ramanpeak center, a melting and crystallizing region (the left hand sideregion with respect to the broken line) and a solid phase orderingregion (the right hand side of the broken line) are observed to bepresent, and the electron mobility increased with lowering oxygenconcentration in the film; similar results were again obtained fornitrogen and carbon atoms. That is, a higher electron mobility wasobtained with decreasing concentration of foreign atoms. Similarly, thesame tendency was observed for hole mobilities.

[0037] Furthermore, the present inventors found that the electronmobility can be intimately correlated with the peak intensity of a peakassigned to the amorphous component within the film. FIG. 4 is a graphshowing the relation between a ratio which the present inventors defineas a peak intensity ratio (Ia/Ic; taken as the abscissa), and theelectron mobility for laser-annealed silicon films with varying oxygenconcentration, where Ia represents the Raman peak intensity (of the peakat a wavenumber of about 480 cm⁻¹) for the amorphous component of thelaser-annealed silicon film, and Ic represents the Raman peak intensity(at about 521 cm⁻¹) for the single crystal silicon. With respect to thefilms in falling in the solid phase ordering region (the right hand sideof the broken line), the electron mobility increased with decreasingoxygen concentration of the film. A similar tendency was observed on theindividual effect of nitrogen and carbon concentrations. Furthermore,the same tendency was observed for hole mobility. The present inventorsdenote again this region at the left hand side of the broken lines shownin FIG. 4 as the melting and crystallizing region, as was the case inFIGS. 2 and 3.

[0038] In conclusion from the foregoing description, a higher carriermobility can be obtained by decreasing the amounts of oxygen, nitrogen,and carbon incorporated into the film. More specifically, the presentinventors have found that, by reducing concentration of each of theforeign atoms to 5×10¹⁹ atoms·cm⁻³ or lower, more preferably to 1×10¹⁹atoms·cm⁻³ or lower, a high electron mobility can be obtained at amaximum probability of a high 80%, provided that the film is subjectedto a less speculative solid phase ordering process which give a higherproduct yield, instead of a melting process. More specifically, it wasfound that by the process above, an electron mobility of 50 cm²/V·s orhigher can be obtained at a maximum probability of 80% on a silicon filmby lowering the concentration of the foreign elements to 5×10¹⁹atoms·cm⁻³ or lower, and that this can be further increased to a high100 cm²/V·s by decreasing the foreign element concentration to 1×10¹⁹atoms·cm⁻³. Furthermore, it was possible to obtain stably a holemobility of from 30 to 80 cm²/V·s by a process similar to above.

[0039] As described in the foregoing, the carrier mobility of a filmhaving brought into a particular solid phase ordering state (which isdenoted as “semi-amorphous state” by the present inventors) can beincreased by decreasing the concentrations of foreign atoms havingincorporated in the film. To achieve this semi-amorphous state, it isrequisite that the film does not undergo melting. Thus, in a process oflong duration, the laser-irradiated portion must be maintained at atemperature not higher than the melting point of the semiconductor. Inthe case of silicon, for example, it requires heating at a temperaturenot higher than 1400° C. under the atmospheric pressure; in germanium,it should be heated to not higher than 1000° C. under the atmosphericpressure. However, in an extremely short period of, for example, 10 nsecas practiced in excimer lasers, a temperature as high as 2000° C. oreven higher is achieved instantaneously and spectroscopically observed.Such a high temperature does not always cause melting of the film. Thus,a definition of a temperature in this case does not make sense.

[0040] In the present invention, the laser annealing is carried out inan atmosphere under atmospheric pressure or reduced pressure.

[0041] For example, a laser beam or a light equivalent to the laser beamis irradiated to an amorphous semiconductor containing therein carbon,nitrogen and oxygen at a concentration of 5×10¹⁹ atoms·cm⁻³ or less,preferably 1×10¹⁹ atoms·cm⁻³ or less, respectively to make order of theamorphous semiconductor higher.

[0042] As mentioned earlier, a high mobility can be obtained effectivelyby reducing the concentration of foreign atoms. In practice, however, itis difficult to control the concentration of the foreign atoms to avalue as low as 1×10¹⁶ atoms·cm⁻³ and below, even if the laser annealingwere to be carried out under an extremely high vacuum to a filmcontaining the atoms at a very low concentration of 1×10¹⁶ atoms·cm⁻³ orlower. This is because the oxygen gas, nitrogen gas, water molecules,carbon dioxide gas, etc., which are present in the atmosphere in traceamounts are taken up in the film during the laser annealing. Otherwise,it is presumably due to the presence of a gas having adsorbed on thesurface of the film, which is then trapped into the film during thelaser annealing.

[0043] To circumvent the aforementioned difficulties, a particularfabrication process is requisite. One of such processes is to firstcover the surface of an amorphous semiconductor film containing foreignatoms of oxygen, nitrogen, and carbon at an extremely low concentrationof 10¹⁵ atoms·cm⁻³ or lower with a protective film such as of siliconoxide, silicon nitride, and silicon carbide, and then laser annealingthe film under a high vacuum at a pressure of 10⁻⁴ Torr or lower. Such aprocess enables a semiconductor film having very high mobility, withextremely low concentrations for oxygen, nitrogen, and carbon. Forexample, a silicon film having an electron mobility of 300 cm²/V·s wasobtained with each of the concentrations of carbon, nitrogen, and oxygenbeing controlled to 1×10¹⁵ atoms·cm⁻³ or lower.

[0044] The protective films, such as of silicon oxide, silicon nitride,and silicon carbide, can be favorably deposited on the surface of anamorphous semiconductor film by continuously depositing the amorphousfilm and the protective film in a same chamber. More specifically, apreferred process comprises, for example, depositing an amorphoussemiconductor film in a chamber equipped with a single vacuum apparatus,using a chemical vapor deposition (CVD) process or a sputtering process,and then, in the same chamber, continuously depositing the protectivefilm while maintaining the same previous atmosphere or by onceevacuating the chamber to an extreme vacuum and then controlling theatmosphere to a one pertinent for the deposition of the protective film.However, to further improve the yield, the reproducibility, and thereliability of the products, it is preferred that independent chambersare provided so that the films are deposited separately therein, andthat the amorphous film once deposited in a particular chamber istransferred to another chamber while maintaining the high vacuum. Theselection of a particular film deposition process depends on the plantand equipment investment. At any rate, the important points to beassured are to sufficiently reduce the amount of oxygen, nitrogen, andcarbon in the film and to avoid adsorption of gases on the surface ofthe amorphous semiconductor and at the boundary between saidsemiconductor and the protective film provided thereon. Even if anamorphous semiconductor film of high purity and a silicon nitrideprotective film thereon were to be formed, the carrier mobility would beimpaired if the amorphous film were to be once exposed to theatmosphere. In general, such an amorphous film once exposed to air wouldyield a low carrier mobility even after laser annealing, and moreover,the probability to obtain a high mobility would be very low. This isbelieved due to the gas adsorbed on the surface of the amorphoussemiconductor film which later diffuses into the film during the laserannealing process.

[0045] The protective film may be made from any material capable oftransmitting a laser beam or a light equivalent to the laser beam, andusable are silicon oxide, silicon nitride, silicon carbide, and amaterial comprising a mixture thereof, which is represented by thechemical formula SiN_(x)O_(y)C_(z) (where, 0≦x≦4/3; 0≦y≦2; 0≦z≦1; and0≦3x+2y+4z≦4). Preferably, the thickness of the film is in the range offrom 5 to 1000 nm.

[0046] In the foregoing description, it has been shown that asemiconductor film having a high carrier mobility can be obtained byreducing the concentrations of oxygen, nitrogen, and carbon of theamorphous semiconductor film and during the laser annealing process. Theelectron mobility and the hole mobility obtained therefrom are mereaverage values of the channel-forming area in the field effecttransistors fabricated for the measurement purposes. Thus, those valuesfail to give the individual mobilities at particular minute portionswithin the channel-forming area. However, as was described referring toFIGS. 1 to 4 according to the present invention, it has been elucidatedthat the carrier mobility can be univocally defined from the parameterssuch as the wavenumber of the Raman peak center, the FWHM of the Ramanpeak, and the intensity of the Raman peak attributed to the amorphouscontent in contrast to that of the whole Raman peak. Thus, by theinformation available from Raman spectroscopy, it is possible tosemi-quantitatively obtain the mobility of a minute area to which adirect measurement cannot be applied.

[0047]FIG. 5 is a graph showing a position-dependent change of FWHM ofthe Raman peak for laser-annealed channel-forming areas of field effecttransistors whose electron mobilities were found to be 101 cm²/V·s inthe case of a semi-amorphous silicon obtained through a solid phaseordering process, and 201 cm²/V·s in the case of a film obtained throughmelting. Referring to FIG. 5, the abscissa is the position of thechannel forming area, X/L, with L being the channel length of 100 μm,and X being the coordinate of the channel forming area. Thus, X/L=0corresponds to the boundary between the channel-forming area and thesource area, X/L=1 represents the boundary between the channel formingarea and the drain area, and X/L=0.5 indicates the center of thechannel-forming area. It can be seen from FIG. 5 that the field effecttransistor having an electron mobility of 101 cm²/V·s has a large FWHMwith little fluctuation. The data is in agreement with the previousobservations of the present authors referring to FIG. 3, i.e., that thecrystallinity approaches to that of a single crystal with decreasingFWHM, and that thereby the electron mobility is increased with reducingFWHM. These data show, in addition, that the positional variation inFWHM (dependence of FWHM on position) is low and hence a uniformcrystallinity is obtained over the whole film. The film yielded anoxygen concentration of about 1×10¹⁹ atoms·cm⁻³.

[0048] The channel-forming area of a field effect transistor having anelectron mobility of 201 cm²/V·s also yielded an oxygen concentration of1×10¹⁹ atoms·cm⁻³. As shown in FIG. 5, the FWHM is lowered over thewhole area, and is largely dependent on the position. Moreover, somepoints yielded FWHM values comparable to, or even lower than that (4.5cm⁻¹) of a single crystal silicon. Such a low FWHM is suggestive of ahigh electron mobility well comparable to that of a single crystalsilicon at that point, and that a localized portion having a highcrystallinity equal to that of a single crystal silicon is present. Fromthe viewpoint of mass-production of devices, such a material havinglarge local fluctuations is not favorable despite a high mobility. Inthe fabrication of compact devices, in particular, the inhomogeneity,which in larger devices was averaged and hence not problematic, becomesmore apparent to considerably reduce the product yield.

[0049] In contrast to above, the film having an electron mobility of 101cm²/V·s yields, as is shown in FIG. 5, that the FWHM generally has lesspositional dependence. Such a material is suitable for the massproduction of devices with improved yield.

[0050] To achieve a high carrier mobility, not only the concentration ofthe foreign atoms should be lowered, but also the conditions for thelaser annealing have to be optimized. The conditions for the laserannealing vary depending on the operating conditions of the laser (suchas whether the laser is operated in a continuous mode or in a pulsedmode, the repetition cycle, beam intensity, and wavelength), and thecoating. The lasers to be used in the process is selected fromultraviolet(UV)-emitting lasers such as the various types of excimerlasers, and lasers emitting light in the visible and infrared (IR)regions, such as a YAG laser. A suitable laser should be selecteddepending on the thickness of the film to be laser-annealed. Inmaterials such as silicon and germanium, the laser annealing is effectedat relatively shallow portions on the surface when a UV-emitting laseris used, because the materials have a short absorption length for UVlight. On the other hand, because those materials have a longerabsorption length for a light in the visible and IR regions, such a beampenetrate deep into the material to effect laser annealing of the innerportions.

[0051] In addition, a further annealing of a laser-annealed asemiconductor film in hydrogen gas atmosphere in the temperature rangeof from 200 to 600° C. for a duration of from 10 minutes to 6 hours waseffective to obtain a high carrier mobility at an improvedreproducibility. This is presumably due to the dangling bonds whichremain in the amorphous area or to the newly formed dangling bonds areaswithin the semiconductor, during the laser annealing for the solid phaseordering. Such dangling bonds then function as barriers to the carriers.If a considerable amount of oxygen, nitrogen, and carbon atoms were tobe incorporated in the semiconductor, those foreign atoms may enter intothe interstices of such dangling bonds; however, when the concentrationsof oxygen, nitrogen, and carbon are extremely low as in the presentcase, the dangling bonds remain as they are and therefore require afurther annealing in hydrogen atmosphere after the laser annealing.

[0052] The present invention is described in further detail belowreferring to non-limiting examples.

EXAMPLE 1

[0053] A planar TFT was fabricated and the electric characteristicsthereof were evaluated. Referring to FIG. 6, the process for thefabrication thereof is described. First, an amorphous silicon film (anactivation layer of the TFT) was deposited on a quartz substrate 601 ata thickness of about 100 nm by a conventional RF sputtering process, ata substrate temperature of 150° C. under an atmosphere consistingsubstantially of 100% argon at a pressure of 0.5 Pa. A 99.99% or higherpurity argon gas was used alone, without adding other gases such ashydrogen. The sputtering was conducted at a power input of 200 W and anRF frequency of 13.56 MHz. The resulting film was etched into a 100μm×500 μm rectangle to obtain a desired amorphous silicon film 602.

[0054] The film was confirmed by secondary ion mass spectroscopy (SIMS)to contain each of oxygen, nitrogen, and carbon at a concentration of10¹⁹ atoms·cm⁻³ or less.

[0055] The film was then placed in a vacuum vessel controlled to avacuum of 10⁻⁵ Torr for laser annealing. A KrF excimer laser wasoperated at a pulse duration of 10 nsec, an irradiation energy of 200mJ, and a pulse repetition of 50 shots, to effect laser annealing byirradiating a laser beam at a wavelength of 248 nm to the film.

[0056] A gate dielectric 603 was then deposited by sputtering on thesurface of the thus obtained laser-annealed silicon film in an oxygenatmosphere, at a thickness of about 100 nm. The film-deposition wasconducted at a substrate temperature of 150° C. and at an RF (13.5 MHz)power input of 400 W. The atmosphere was controlled to be substantiallyoxygen, and no other gas was intentionally added. The oxygen gas was of99.9% or higher purity, and the pressure thereof was 0.5 Pa.

[0057] Then, a 200 nm thick aluminum film was deposited by a knownvacuum deposition process, and was further subjected to a conventionaldry etching process to remove the unnecessary portions to obtain a 100μm wide gate 604. At this point, a photoresist 605 used in the dryetching process was left unremoved on the gate.

[0058] Boron ions were then doped to the whole structure other than thegate by ion implantation at a dopant concentration of 10¹⁴ cm⁻². In thiscase, the gate and the photoresist thereon were utilized as masks toavoid doping of boron ions to the portion under the gate. Thus wereobtained impurity areas, i.e., a source area 606 and a drain area 607.The resulting structure is given in FIG. 6(B).

[0059] The resulting structure was then placed in a vacuum vesselcontrolled to a vacuum of 10⁻⁵ Torr for laser annealing. A KrF excimerlaser was operated at a pulse duration of 10 nsec, an irradiation energyof 100 mJ, and a pulse repetition of 50 shots, to effect the filmannealing by irradiating a beam at a wavelength of 248 nm. The impurityareas which were made amorphous by the ion doping were thusrecrystallized.

[0060] The laser-annealed structure was thermally annealed further inhydrogen atmosphere. The substrate was placed in a chamber equipped witha vacuum-evacuating means. The chamber was first evacuated to a vacuumof 10⁻⁶ Torr using a turbo molecular pump, and this state was maintainedfor 30 minutes. Then a 99.99% or higher purity hydrogen gas wasintroduced to the chamber until the pressure was recovered to 100 Torr,to anneal the substrate at 300° C. for 60 minutes. The vessel was onceevacuated to remove the adhered gases, water, and the like from thefilm, because it had been known empirically that a high mobility withfavorable reproducibility cannot be obtained for the films havingthermally annealed with those impurities being adhered thereto.

[0061] Finally, a 100 nm thick silicon oxide film provided on the top ofthe source and the drain areas was perforated to form thereon aluminumcontacts 608 and 609. Thus was a complete field effect transistorobtained. A channel is located in the activation layer under the gateelectrode between the source and the drain areas in the field effecttransistor and comprises the intrinsic or substantially intrinsicsemiconductor crystallized by the KrF excimer laser.

[0062] The measurement of C-V characteristics on this field effecttransistor yielded an electron mobility of 98 cm²/V·s for the channelforming area. The threshold voltage was 4.8 V. The concentrations ofoxygen, nitrogen, and carbon of this field effect transistor were eachfound by SIMS to be 1×10¹⁹ atoms·cm⁻³ or lower.

EXAMPLE 2

[0063] A planar TFT was fabricated and the electric characteristicsthereof were evaluated. First, an amorphous silicon film containingphosphorus at a concentration of 3×10¹⁷ atoms·cm⁻³ was deposited on aquartz substrate at a thickness of about 100 nm by a conventional RFsputtering process. The amorphous silicon film having deposited at thisthickness can be wholly annealed by a KrF laser beam at a wavelength of248 nm. The RF sputtering was performed at a substrate temperature of150° C. under an atmosphere consisting substantially of 100% argon at apressure of 0.5 Pa. A 99.99% or higher purity argon gas was used alone,without adding other gases such as hydrogen. The sputtering wasconducted at a power input of 200 W and an RF frequency of 13.56 MHz.The resulting film was etched into a 100 μm×500 μm rectangle to obtainthe desired amorphous silicon film.

[0064] The film was confirmed by secondary ion mass spectroscopy (SIMS)to contain each of oxygen, nitrogen, and carbon at a concentration of10¹⁹ atoms·cm⁻³ or less.

[0065] A gate dielectric was then deposited by sputtering on the surfaceof the thus obtained silicon film in an oxygen atmosphere, at athickness of about 100 nm. The film-deposition was conducted at asubstrate temperature of 150° C. and at an RF (13.56 MHz) power input of400 W. The atmosphere was controlled to be substantially oxygen, and noother gas was intentionally added. The oxygen gas was of 99.9% or higherpurity, and the pressure thereof was 0.5 Pa.

[0066] Then, a 200 nm thick aluminum film was deposited by a knownvacuum deposition process, and was further subjected to a conventionaldry etching process to remove the unnecessary portions to obtain a gateat a width of 100 μm. At this point, a photoresist used in the dryetching process was left on the gate.

[0067] Boron ions were then doped to the whole structure except for thegate by ion implantation at a dopant concentration of 10¹⁴ cm⁻². In thiscase, the gate and the photoresist thereon were utilized as masks toavoid doping of boron ions to the portion under the gate. Thus wereobtained impurity areas, i.e., a source area and a drain area, in thesilicon film.

[0068] The resulting structure was then placed in a vacuum vesselcontrolled to a vacuum of 10⁻⁵ Torr for laser annealing. A KrF excimerlaser was operated at a pulse duration of 10 nsec, an irradiation energyof 100 mJ, and a pulse repetition of 50 shots, to anneal the film byirradiating a beam at a wavelength of 248 nm from the back of thesubstrate. Thus was the amorphous silicon film converted into asemi-amorphous film. This process is different from that of EXAMPLE 1 inthat the source or the drain area is imparted semi-amorphoussimultaneously with the channel forming area. The instant process istherefore advantageous in that a favorable boundary with a continuouscrystallinity can be obtained with less defects, as compared with theprocess of EXAMPLE 1 in which many defects were found to generate in theboundary between the source or drain area and the channel forming area.

[0069] The laser-annealed structure was thermally annealed further inhydrogen atmosphere. The substrate was placed in a chamber equipped witha vacuum-evacuating means. The chamber was first evacuated to a vacuumof 10⁻⁶ Torr using a turbo molecular pump, and then heated to 100° C. tomaintain this state for 30 minutes. Then a 99.99% or higher purityhydrogen gas was introduced to the chamber until the pressure wasrecovered to 100 Torr, so that annealing of the substrate may beconducted then at 300° C. for 60 minutes. The vessel was once evacuatedto remove the adhered gases, water, and the like from the film, becauseit had been known empirically that a high mobility with favorablereproducibility cannot be obtained for the films having thermallyannealed with those impurities being adhered thereto.

[0070] Finally, a 100 nm thick silicon oxide film provided on the top ofthe source and the drain areas was perforated to form thereon aluminumcontacts. Thus was the structure completed into a field effecttransistor.

[0071] The measurement of C-V characteristics on this field effecttransistor yielded an electron mobility of 112 cm²/V·s for the channelforming area. The threshold voltage was 3.9 V. The present field effecttransistor yielded an improved (lower) threshold voltage as comparedwith that of the field effect transistor fabricated in EXAMPLE 1. Thisis assumed attributable to the simultaneous laser annealing of theimpurity areas and the channel forming area, which might havecrystallized uniformly at the same time by irradiating the laser beamfrom the back. Furthermore, the drain current ratio at the ON/OFF of thegate voltage was found to be 5×10⁶.

[0072] The concentrations of oxygen, nitrogen, and carbon of this fieldeffect transistor were each found by SIMS to be 1×10¹⁹ atoms·cm⁻³ orlower. The Raman spectroscopy of the channel forming area resolved aRaman peak at a center wavenumber of 515 cm⁻¹, having a FWHM of 13 cm⁻¹.The presence of a once melted and recrystallized silicon was notobserved, and the development of a semi-amorphous state was confirmed.

EXAMPLE 3

[0073] A planar TFT was fabricated and the electric characteristicsthereof were evaluated. First, about 100 nm thick amorphous silicon filmand a 10 nm thick silicon nitride coating thereon were continuouslydeposited on a quartz substrate having coated with a 10 nm thick siliconnitride coating by using a film-deposition apparatus having twochambers. The amorphous silicon film was deposited by a conventionalsputtering method, and the silicon nitride film was deposited by aglow-discharge plasma chemical vapor deposition (CVD).

[0074] The substrate was set in a first pre-chamber which was heated to200° C. and evacuated to a pressure of 10⁻⁶ Torr or lower for 1 hour.Separately, an air-tight first chamber, which is constantly controlledto a pressure of 10⁻⁴ Torr or lower except for the case of filmdeposition, was evacuated to 10⁻⁶ Torr. The substrate was transferredfrom the first pre-chamber to the first chamber and set therein, atwhich point the chamber was evacuated to 10⁻⁶ Torr or lower whilemaintaining the substrate and the target to a temperature of 200° C. fora duration of 1 hour. Then, argon gas was introduced into the chamber togenerate an RF plasma to conduct film deposition by sputtering. A99.9999% or higher purity silicon target containing 1 ppm of phosphoruswas used for the target. The film deposition was conducted with thesubstrate temperature being maintained at 150° C., in an atmosphereconsisting substantially of 100% argon gas at a pressure of 5×10⁻² Torr.No gases other than argon, such as hydrogen, was added intentionally.The argon gas used herein was of 99.9999% or higher purity. Thesputtering was operated at an input power of 200 W and an RF frequencyof 13.56 MHz.

[0075] Upon completion of the film deposition, the RF discharge was cutoff, and while evacuating the first chamber to a vacuum of 10⁻⁶ Torr, anair-tight second pre-chamber, which is provided between the first andsecond chambers and is constantly maintained to a pressure of 10⁻⁵ Torror lower, was vacuum-evacuated to 10⁻⁶ Torr, so that the substrate maybe transferred therein from the first chamber. Then, an air-tight secondchamber, which is always maintained at a pressure of 10⁻⁴ Torr or lowerexcept for the case of carrying out a film deposition, was evacuated to10⁻⁶ Torr to set therein the substrate having transferred from thesecond pre-chamber. The substrate was kept at 200° C. in the secondchamber, while the chamber was evacuated to maintain the substrate undera pressure of 10⁻⁶ Torr or lower for 1 hour.

[0076] Then, a gas mixture diluted with hydrogen and comprising a99.9999% or higher purity ammonia gas and disilane (Si₂H₆) gas at aratio of 3:2 was introduced in the second chamber to control the overallpressure to 10⁻¹ Torr. An RF current was applied to the chamber togenerate a plasma therein, so that a silicon nitride film might bedeposited on the substrate. The power input was 200 W, at a frequency of13.56 MHz.

[0077] After completion of the film deposition, the RF discharge was cutoff. While evacuating the second chamber to 10⁻⁶ Torr, a thirdpre-chamber, which is provided at one side of the second chamber andhaving a quartz window, was vacuum evacuated to 10⁻⁶ Torr, at whichpoint the substrate was transferred from the second chamber to the thirdpre-chamber. Then, a KrF excimer laser was operated at a pulse durationof 10 nsec, an irradiation energy of 100 mJ, and a pulse repetition of50 shots, so that a laser beam at a wavelength of 248 nm was irradiatedto the film to effect the laser annealing. Thus was a semi-amorphousfilm obtained from the amorphous silicon film.

[0078] The process described above was particularly effective forimproving the product yield. By thus conducting continuously the laserannealing from the point of film deposition without substantiallydisturbing the vacuum state, a high product yield can be achievedirrespective of the presence of a protective film on the amorphous film.Accordingly, the process is effective on the FETs described in EXAMPLES1 and 2 as well, in which no protective films are provided. Assumably,the films are maintained free from adhesion of dusts and the like whichfunction as nuclei for crystal growth, and from scratches and otherdefects; moreover, they are free from adsorption of water and gaseswhich readily allow development of polycrystalline materials.Furthermore, such a process prevents exertion of a non-uniform stress onthe film at the transfer from a vacuum state to a pressurized stateunder the atmosphere. Such a non-uniform stress causes microscopicchange, protrusions, and the like to form on the surface of the film,which function as nuclei to develop a polycrystalline structure.

[0079] The process of continuously conducting the film deposition andthe annealing thereof as described above, may be carried out in twoways. One is establishing a film-deposition chamber and separately apre-chamber as described in the foregoing, and providing a window in thepre-chamber to effect the laser annealing through this window. The otherprocess is similar to the first, except that the window is provided tothe film-deposition chamber so that the laser annealing may be effectedsubsequently to the film deposition. The latter process, however,requires etching of the window to remove the coating having adheredduring the film-deposition process, since the coating adhesions make thewindow translucent. Accordingly, the former process is favorable fromthe viewpoint of its applicability to mass production and themaintenance costs.

[0080] After the laser annealing is finished in the third pre-chamber, adry nitrogen gas is introduced into the third pre-chamber to recover theatmospheric pressure. The substrate was then taken out from the thirdpre-chamber, and the silicon nitride film having deposited thereon wasremoved by a known dry etching process. The resulting silicon film wasetched into a 100 μm×500 μm rectangular shape.

[0081] The concentrations of oxygen, nitrogen, and carbon of this fieldeffect transistor were each 1×10¹⁶ atoms·cm⁻³ or lower, which were eachconfirmed by SIMS performed on a separate film fabricated by the sameprocess.

[0082] A gate dielectric was then deposited on the surface of the thusobtained laser-annealed silicon film in an oxygen atmosphere bysputtering, at a thickness of about 100 nm. The film-deposition wasconducted at a substrate temperature of 150° C. and at an RF (13.56 MHz)power input of 400 W. A 99.9999% or higher purity silicon oxide was usedas the target for sputtering. The atmosphere was controlled to besubstantially oxygen, and no other gas was intentionally added. Theoxygen gas was of 99.999% or higher purity, and the pressure thereof was5×10⁻² Torr.

[0083] Then, a 200 nm thick aluminum film was deposited by a knownvacuum deposition process, and was further subjected to a conventionaldry etching process to remove the unnecessary portions to obtain a gateat a width of 100 μm. At this point, a photoresist used in the dryetching process was left on the gate.

[0084] Boron ions were then doped to the whole structure except for thegate by ion implantation at a dopant concentration of 10¹⁴ cm⁻². In thiscase, the gate and the photoresist thereon were utilized as masks toavoid doping of boron ions to the portion under the gate. Thus wereobtained impurity areas, i.e., a source area and a drain area, in thesilicon film.

[0085] The resulting structure was then placed in a vacuum vesselcontrolled to a vacuum of 10⁻⁵ Torr for laser annealing. A KrF excimerlaser was operated at a pulse duration of 10 nsec, an irradiation energyof 50 mJ, and a pulse repetition of 50 shots, to anneal the film byirradiating a beam at a wavelength of 248 nm from the back of thesubstrate. Thus was a semi-amorphous film obtained from the silicon filmhaving imparted amorphous by ion-doping.

[0086] The present process is similar to that described in EXAMPLE 1 inthe point that the laser annealing is conducted in two steps, however,the present process is distinguishable from the previous in that thelaser annealing is effected by irradiating the beam from the back of thesubstrate to form a continuous junction between the impurity regions andthe channel forming area. Because despite a UV-emitting laserefficiently anneals the surface of the film, it frequently fails toanneal the portions deep inside the film, laser beam is irradiated firstfrom the surface of the amorphous silicon film, and then from the backof the substrate, to obtain a continuous junction between the channelforming region and the impurity regions and to improve product yield.

[0087] Laser annealing by irradiation of an ultraviolet laser beam fromthe substrate surface is effective for the surface portion to which theultraviolet laser beam is irradiated, and is not sufficiently effectivefor deeper portion, that is, there is a high possibility that a highmobility cannot be obtained in the deeper portion. Thereby productionyield may be lowered. Laser irradiation from the back of the substrateis not sufficiently effective for obtaining a high mobility in a portionof the activation layer in contact with the gate electrode. Therefore,laser annealing is carried out twice in this embodiment, that is, thefirst laser annealing is carried out by irradiating a laser beam to theamorphous silicon film from the surface side thereof and the secondlaser annealing is carried out by irradiating a laser beam to the filmfrom the back of the substrate, to increase the production yield and toestablish a continuous junction between the channel forming area and theimpurity areas.

[0088] The laser-annealed structure was thermally annealed further inhydrogen atmosphere. The substrate was placed in a chamber equipped witha vacuum-evacuating means. The chamber was first evacuated to a vacuumof 10⁻⁶ Torr using a turbo molecular pump, and then heated to 100° C. tomaintain this state for 30 minutes. Then a 99.99% or higher purityhydrogen gas was introduced to the chamber until the pressure wasrecovered to 100 Torr, so that annealing of the substrate may beconducted then at 300° C. for 60 minutes. The vessel was once evacuatedto remove the adhered gases, water, and the like from the film, becauseit had been known empirically that a high mobility with favorablereproducibility cannot be obtained for the films having thermallyannealed with those impurities being adhered thereto.

[0089] Finally, a 100 nm thick silicon oxide film provided on the top ofthe source and the drain areas was perforated to form thereon aluminumcontacts. Thus was the structure completed into a field effecttransistor.

[0090] One hundred field effect transistors above were fabricated. Themeasurement of C-V characteristics on these field effect transistorsyielded an average electron mobility of 275 cm²/V·s for the channelforming area. The threshold voltage was 4.2 V in average. The draincurrent ratio at the ON/OFF of the gate voltage was found to be 8×10⁶ inaverage. Each of the field effect transistors thus fabricated waschecked whether it had favorable characteristics on electron mobility,threshold voltage, and drain current ratio. The standard values for theelectron mobility, the threshold voltage, and the drain current ratiowere set to 100 cm²/V·s, 5.0 V, and 1×10⁶, respectively. Eighty-one outof 100 field effect transistors were found as being favorable.

[0091] The concentrations of oxygen, nitrogen, and carbon of these fieldeffect transistors evaluated as favorable were each found by SIMS to be1×10¹⁶ atoms·cm⁻³ or lower.

EXAMPLE 4

[0092] A planar TFT was fabricated and the electric characteristicsthereof were evaluated. First, about 100 nm thick amorphous silicon filmand a 10 nm thick silicon nitride coating thereon were continuouslydeposited on a quartz substrate having coated with a 10 nm thick siliconnitride coating by using a film-deposition apparatus having twochambers. The amorphous silicon film was deposited by a conventionalsputtering method, and the silicon nitride film was deposited by aglow-discharge plasma chemical vapor deposition (CVD).

[0093] The substrate was set in a first pre-chamber which was heated to200° C. and evacuated to a pressure of 10⁻⁶ Torr or lower for 1 hour.Separately, an air-tight first chamber, which is constantly controlledto a pressure of 10⁻⁴ Torr or lower except for the case of filmdeposition, was evacuated to 10⁻⁶ Torr. The substrate was transferredfrom the first pre-chamber to the first chamber and set therein, atwhich point the chamber was evacuated to 10⁻⁶ Torr or lower whilemaintaining the substrate and the target to a temperature of 200° C. fora duration of 1 hour. Then, argon gas was introduced into the chamber togenerate an RF plasma to conduct film deposition by sputtering. A99.9999% or higher purity silicon target containing 1 ppm of phosphoruswas used for the target. The film deposition was conducted with thesubstrate temperature being maintained at 150° C., in an atmosphereconsisting substantially of 100% argon gas at a pressure of 5×10⁻² Torr.No gases other than argon, such as hydrogen, was added intentionally.The argon gas used herein was of 99.9999% or higher purity. Thesputtering was operated at an input power of 200 W and an RF frequencyof 13.56 MHz.

[0094] Upon completion of the film deposition, the RF discharge was cutoff, and while evacuating the first chamber to a vacuum of 10⁻⁶ Torr, anair-tight second pre-chamber, which is provided between the first andsecond chambers and is constantly maintained to a pressure of 10⁻⁵ Torror lower, was vacuum-evacuated to 10⁻⁶ Torr, so that the substrate maybe transferred therein from the first chamber. Then, an air-tight secondchamber, which is always maintained at a pressure of 10⁻⁴ Torr or lowerexcept for the case of carrying out a film deposition, was evacuated to10⁻⁶ Torr to set therein the substrate having transferred from thesecond pre-chamber. The substrate was kept at 200° C. in the secondchamber, while the chamber was evacuated to maintain the substrate undera pressure of 10⁻⁶ Torr or lower for 1 hour.

[0095] Then, a gas mixture diluted with hydrogen and comprising a99.9999% or higher purity ammonia gas and disilane (Si₂H₆) gas at aratio of 3:2 was introduced in the second chamber to control the overallpressure to 10⁻¹ Torr. An RF current was applied to the chamber togenerate a plasma therein, so that a silicon nitride film might bedeposited on the substrate. The power input was 200 W, at a frequency of13.56 MHz.

[0096] After completion of the film deposition, the RF discharge was cutoff. While evacuating the second chamber to 10⁻⁶ Torr, a thirdpre-chamber, which is provided at one side of the second chamber andhaving a quartz window, was vacuum evacuated to 10⁻⁶ Torr, at whichpoint the substrate was transferred from the second chamber to the thirdpre-chamber. Then, argon gas having a purity of 99.9999% or higher wasintroduced into the third pre-chamber to a pressure of 5 atm. Then, aKrF excimer laser was operated at a pulse duration of 10 nsec, anirradiation energy of 100 mJ, and a pulse repetition of 50 shots, sothat a laser beam at a wavelength of 248 nm was irradiated to the filmto effect the laser annealing. Thus was a semi-amorphous film obtainedfrom the amorphous silicon film.

[0097] The process described above was particularly effective forimproving the product yield. By thus conducting continuously the laserannealing from the point of film deposition without substantiallydisturbing the vacuum state, a high product yield can be achievedirrespective of the presence of a protective film on the amorphous film.Accordingly, the process is effective on the FETs described in EXAMPLES1 and 2 as well, in which no protective films are provided. Assumably,the films are maintained free from adhesion of dusts and the like whichfunction as nuclei for crystal growth, and from scratches and otherdefects. Furthermore, in the process according to this EXAMPLE, thelaser annealing conducted under a pressurized condition avoidsgeneration of microscopic bubbles inside the film, and thereby preventsthe film to develop a polycrystalline structure, because such bubblesserve as nuclei for the formation of such a structure.

[0098] The process of continuously conducting the film deposition andthe annealing thereof as described above, may be carried out in twoways. One is establishing a film-deposition chamber and separately apre-chamber as described in the foregoing, and providing a window in thepre-chamber to effect the laser annealing through this window. The otherprocess is similar to the first, except that the window is provided tothe film deposition chamber so that the laser annealing may be effectedsubsequently to the film deposition. The latter process, however,requires etching of the window to remove the coating having adheredduring the film-deposition process, since the coating adhesions make thewindow translucent. Accordingly, the former process is favorable fromthe viewpoint of its applicability to mass production and themaintenance costs.

[0099] After the laser annealing is finished in the third pre-chamber, adry nitrogen gas is introduced into the third pre-chamber to recover theatmospheric pressure. The substrate was then taken out from the thirdpre-chamber, and the silicon nitride film having deposited thereon wasremoved by a known dry etching process. The resulting silicon film wasetched into a 10 μm×1 μm rectangular shape.

[0100] The concentrations of oxygen, nitrogen, and carbon of this fieldeffect transistor were each 1×10¹⁶ atoms·cm⁻³ or lower, which were eachconfirmed by SIMS performed on a separate film fabricated by the sameprocess.

[0101] A gate dielectric was then deposited on the surface of the thusobtained laser-annealed silicon film in an oxygen atmosphere bysputtering, at a thickness of about 100 nm. The film-deposition wasconducted at a substrate temperature of 150° C. and at an RF (13.56 MHz)power input of 400 W. A 99.9999% or higher purity silicon oxide was usedas the target for sputtering. The atmosphere was controlled to besubstantially oxygen, and no other gas was intentionally added. Theoxygen gas was of 99.999% or higher purity, and the pressure thereof was5×10⁻² Torr.

[0102] Then, a 200 nm thick aluminum film was deposited by a knownvacuum deposition process, and was further subjected to a conventionaldry etching process to remove the unnecessary portions to obtain a gateat a gate electrode width (channel length) of 0.5 μm and at a channelwidth of 1 μm. At this point, a photoresist used in the dry etchingprocess was left on the gate.

[0103] Boron ions were then doped to the whole structure except for thegate by ion implantation at a dopant concentration of 10¹⁴ cm². In thiscase, the gate and the photoresist thereon were utilized as masks toavoid doping of boron ions to the portion under the gate. Thus wereobtained impurity areas, i.e., a source area and a drain area, in thesilicon film.

[0104] The resulting structure was then placed in a vacuum vesselcontrolled to a vacuum of 10⁻⁵ Torr for laser annealing. A KrF excimerlaser was operated at a pulse duration of 10 nsec, an irradiation energyof 50 mJ, and a pulse repetition of 50 shots, to anneal the film byirradiating a beam at a wavelength of 248 nm from the back of thesubstrate. Thus was a semi-amorphous film obtained from the silicon filmhaving imparted amorphous by ion-doping.

[0105] The present process is similar to that described in EXAMPLE 1 inthe point that the laser annealing is conducted in two steps, however,the present process is distinguishable from the previous in that thelaser annealing is effected by irradiating the beam from the back of thesubstrate to form a continuous junction between the impurity regions andthe channel forming area. Because despite a UV-emitting laserefficiently anneals the surface of the film, it frequently fails toanneal the portions deep inside the film, laser beam is irradiated firstfrom the surface of the amorphous silicon film, and then from the backof the substrate, to obtain a continuous junction between the channelforming region and the impurity regions and to improve product yield.

[0106] Laser annealing by irradiation of an ultraviolet laser beam fromthe substrate surface is effective for the surface portion to which theultraviolet laser beam is irradiated, and is not sufficiently effectivefor deeper portion, that is, there is a high possibility that a highmobility cannot be obtained in the deeper portion. Thereby productionyield may be lowered. Laser irradiation from the back of the substrateis not sufficiently effective for obtaining a high mobility in a portionof the activation layer in contact with the gate electrode. Therefore,laser annealing is carried out twice in this embodiment, that is, thefirst laser annealing is carried out by irradiating a laser beam to theamorphous silicon film from the surface side thereof and the secondlaser annealing is carried out by irradiating a laser beam to the filmfrom the back of the substrate, to increase the production yield and toestablish a continuous junction between the channel forming area and theimpurity areas.

[0107] The laser-annealed structure was thermally annealed further inhydrogen atmosphere. The substrate was placed in a chamber equipped witha vacuum-evacuating means. The chamber was first evacuated to a vacuumof 10⁶ ⁻Torr using a turbo molecular pump, and then heated to 100° C. tomaintain this state for 30 minutes. Then a 99.99% or higher purityhydrogen gas was introduced to the chamber until the pressure wasrecovered to 100 Torr, so that annealing of the substrate may beconducted then at 300° C. for 60 minutes. The vessel was once evacuatedto remove the adhered gases, water, and the like from the film, becauseit had been known empirically that a high mobility with favorablereproducibility cannot be obtained for the films having thermallyannealed with those impurities being adhered thereto.

[0108] Finally, a 100 nm thick silicon oxide film provided on the top ofthe source and the drain areas was perforated to form thereon aluminumcontacts. Thus was the structure completed into a field effecttransistor.

[0109] One hundred field effect transistors above were fabricated. Themeasurement of C-V characteristics on these field effect transistorsyielded an average electron mobility of 259 cm²/V·s for the channelforming area. The threshold voltage was 4.2 V in average. The draincurrent ratio at the ON/OFF of the gate voltage was found to be 8×10⁶ inaverage. Each of the field effect transistors thus fabricated waschecked whether it had favorable characteristics on electron mobility,threshold voltage, and drain current ratio. The standard values for theelectron mobility, the threshold voltage, and the drain current ratiowere set to 100 cm²/V·s, 5.0 V, and 1×10⁶, respectively. Seventy-one outof 100 field effect transistors were found as being favorable. Thisexample illustrates that the present invention is greatly effective forproducing fine devices.

[0110] The concentrations of oxygen, nitrogen, and carbon of these fieldeffect transistors evaluated as favorable were each found by SIMS to be1×10¹⁶ atoms·cm⁻³ or lower.

[0111] As described in the foregoing, the present invention providesthin film semiconductors of high mobility at an excellentreproducibility. The description above was made mainly on the laserannealing of a semiconductor film having deposited on an insulatorsubstrate made of quartz and the like. However, the present invention isnot only limited thereto and can be applied to single crystalsemiconductors such as single crystal silicon substrates which are usedin, for example, monolithic integrated circuits (ICs). There should benoted, however, that it is not favorable to deposit a relatively thinamorphous film, i.e., a film as thin as to which laser annealingeffectively take place, directly on a single crystal or polycrystallinesubstrate. This is because the substrate provides nuclei for theformation of a polycrystalline structure. However, if a sufficientlythick amorphous film were to be deposited on a single crystal or apolycrystalline substrate, the laser annealing has no effect to portionsdeep inside the amorphous film. Thus, a favorable semi-amorphous stateis achieved. Such a problem can be circumvented, of course, if anamorphous material of silicon oxide, silicon nitride, etc., is formed onthe surface of the single crystal or the polycrystalline substrate.

[0112] Furthermore, in addition to the silicon film which was describedin great detail in the EXAMPLES, the present invention can be applied toa germanium film, silicon-germanium alloy films, or films of variousother intrinsic semiconductor materials as well as compoundsemiconductor materials. As described hereinbefore, it should be furthernoted that the term “laser annealing”, which is used as a means ofincreasing the mobility of an amorphous film, refers inclusively tomeans in which a high density energy beam is used, such as a flash lampannealing. Thus, it should be noted that a process using a high densityenergy beam for improving the crystallinity of a semiconductor materialis within the scope of the present invention.

[0113] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed:
 1. A method for manufacturing a semiconductor devicecomprising the steps of: forming a semiconductor film on an insulatingsurface; crystallizing said semiconductor film by irradiating a firstlight to said semiconductor film without melting said semiconductorfilm; introducing an impurity element into a portion of the crystallinesemiconductor film; and activating said impurity element by irradiatinga second light to said crystalline semiconductor film without meltingsaid crystalline semiconductor film.
 2. A method for manufacturing asemiconductor device according to claim 1 , wherein said first light isa laser light or a lamp light.
 3. A method for manufacturing asemiconductor device according to claim 1 , wherein said second light isa laser light or a lamp light.
 4. A method for manufacturing asemiconductor device according to claim 1 , wherein said impurityelement is an N-type impurity element or a P-type impurity element.
 5. Amethod for manufacturing a semiconductor device according to claim l,wherein said semiconductor film contains at least one of carbon, oxygenand a nitrogen at a concentration of 1×10¹⁹ atoms/cm³.
 6. A method formanufacturing a semiconductor device comprising the steps of: forming asemiconductor film on an insulating surface; crystallizing saidsemiconductor film by irradiating a first light to said semiconductorfilm without melting said semiconductor film; introducing an impurityelement into a portion of the crystalline semiconductor film; andactivating said impurity element by irradiating a second light to saidcrystalline semiconductor film without melting said crystallinesemiconductor film, wherein said crystalline semiconductor film shows aRaman shift at a wave number of 517 cm⁻¹ or less.
 7. A method formanufacturing a semiconductor device according to claim 6 , wherein saidfirst light is a laser light or a lamp light.
 8. A method formanufacturing a semiconductor device according to claim 6 , wherein saidsecond light is a laser light or a lamp light.
 9. A method formanufacturing a semiconductor device according to claim 6 , wherein saidimpurity element is an N-type impurity element or a P-type impurityelement.
 10. A method for manufacturing a semiconductor device accordingto claim 6 , wherein said semiconductor film contains at least one ofcarbon, oxygen and a nitrogen at a concentration of 1×10¹⁹ atoms/cm³.11. A method for manufacturing a semiconductor device comprising thesteps of: forming a semiconductor film on an insulating surface;crystallizing said semiconductor film by irradiating a first light tosaid semiconductor film without melting said semiconductor film;introducing an impurity element into a portion of the crystallinesemiconductor film; activating said impurity element by irradiating asecond light to said crystalline semiconductor film without melting saidcrystalline semiconductor film; and annealing said crystallinesemiconductor film in an atmosphere containing hydrogen.
 12. A methodfor manufacturing a semiconductor device according to claim 11 , whereinsaid first light is a laser light or a lamp light.
 13. A method formanufacturing a semiconductor device according to claim 11 , whereinsaid second light is a laser light or a lamp light.
 14. A method formanufacturing a semiconductor device according to claim 11 , whereinsaid impurity element is an N-type impurity element or a P-type impurityelement.
 15. A method for manufacturing a semiconductor device accordingto claim 11 , wherein said semiconductor film contains at least one ofcarbon, oxygen and a nitrogen at a concentration of 1×10¹⁹ atoms/cm³.16. A method for manufacturing a semiconductor device comprising thesteps of: forming a semiconductor film on an insulating surface; forminga protective film on said semiconductor film, said protective filmcomprising a material expressed by a formula SiNxOyCz where 0 x 4/3, 0 y1 and 0 3x+2y+4z 4; crystallizing said semiconductor film by irradiatinga first light to said semiconductor film without melting saidsemiconductor film; introducing an impurity element into a portion ofthe crystalline semiconductor film; and activating said impurity elementby irradiating a second light to said crystalline semiconductor filmwithout melting said crystalline semiconductor film.
 17. A method formanufacturing a semiconductor device according to claim 16 , whereinsaid first light is a laser light or a lamp light.
 18. A method formanufacturing a semiconductor device according to claim 16 , whereinsaid second light is a laser light or a lamp light.
 19. A method formanufacturing a semiconductor device according to claim 16 , whereinsaid impurity element is an N-type impurity element or a P-type impurityelement.
 20. A method for manufacturing a semiconductor device accordingto claim 16 , wherein said semiconductor film contains at least one ofcarbon, oxygen and a nitrogen at a concentration of 1×10¹⁹ atoms/cm³.21. A method for manufacturing a semiconductor device comprising thesteps of: forming a semiconductor film on an insulating surface;crystallizing said semiconductor film by irradiating a first light tosaid semiconductor film without melting said semiconductor film;introducing an impurity element into a portion of the crystallinesemiconductor film; and activating said impurity element by irradiatinga second light to said crystalline semiconductor film without meltingsaid crystalline semiconductor film, wherein the crystallizing step iscontinuously performed after the forming semiconductor film step withoutexposing said semiconductor film to air
 22. A method for manufacturing asemiconductor device according to claim 21 , wherein said first light isa laser light or a lamp light.
 23. A method for manufacturing asemiconductor device according to claim 21 , wherein said second lightis a laser light or a lamp light.
 24. A method for manufacturing asemiconductor device according to claim 21 , wherein said impurityelement is an N-type impurity element or a P-type impurity element. 25.A method for manufacturing a semiconductor device according to claim 21, wherein said semiconductor film contains at least one of carbon,oxygen and a nitrogen at a concentration of 1×10¹⁹ atoms/cm³.
 26. Amethod for manufacturing a semiconductor device comprising the steps of:forming a semiconductor film on an insulating surface; forming aprotective film on said semiconductor film, said protective filmcomprising a material expressed by a formula SiNxOyCz where 0 x 4/3, 0 y1 and 0 3x+2y+4z 4; crystallizing said semiconductor film by irradiatinga first light to said semiconductor film without melting saidsemiconductor film; removing said protective film; forming an insulatingfilm on the crystalline semiconductor film; introducing an impurityelement into a portion of said crystalline semiconductor film throughsaid insulating film; and activating said impurity element byirradiating a second light to said crystalline semiconductor filmwithout melting said crystalline semiconductor film.
 27. A method formanufacturing a semiconductor device according to claim 26 , whereinsaid first light is a laser light or a lamp light.
 28. A method formanufacturing a semiconductor device according to claim 26 , whereinsaid second light is a laser light or a lamp light.
 29. A method formanufacturing a semiconductor device according to claim 26 , whereinsaid impurity element is an N-type impurity element or a P-type impurityelement.
 30. A method for manufacturing a semiconductor device accordingto claim 26 , wherein said semiconductor film contains at least one ofcarbon, oxygen and a nitrogen at a concentration of 1×10¹⁹ atoms/cm³.